#--
#     This file is part of the Prelude library that provides tools to
#     enable Haskell style functional programming in Ruby.
#
#     http://prelude.rubyforge.org 
#     
#     Copyright (C) 2006 APP Design, Inc.
#
#     This library is free software; you can redistribute it and/or
#     modify it under the terms of the GNU Lesser General Public
#     License as published by the Free Software Foundation; either
#     version 2.1 of the License, or (at your option) any later version.
#
#     This library is distributed in the hope that it will be useful,
#     but WITHOUT ANY WARRANTY; without even the implied warranty of
#     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
#     Lesser General Public License for more details.
#
#     You should have received a copy of the GNU Lesser General Public
#     License along with this library; if not, write to the Free Software
#     Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
#++

module Prelude

  # $Id: monad.rb 34 2007-10-23 21:38:09Z prelude $
  #
  # Defines monadic behavior for anything willing to include it.
  module Monad
    M_STATE = {}

    def wrap
      M_STATE[object_id] = self
      self
    end

    def unwrap
      M_STATE[object_id]
    end

    def empty
      M_STATE[object_id] = nil
      self
    end
    
    def Monad.join(arr)
      r = []
      arr.each {|a| r.push *a}
      r
    end

    def bind(f=nil)
      wrap unless M_STATE[object_id]
      if f.kind_of?(Symbol) || f.kind_of?(Method) || f.kind_of?(Proc)
        M_STATE[object_id] = f.to_proc.call(M_STATE[object_id])
        self
      else
        f
      end # if
    end # bind

    alias >> bind

  end # Monad

#   class Maybe < Monad
    
#     # maybe		-- :: b -> (a -> b) -> Maybe a -> b
#     def maybe
#       warn "Method 'maybe' is not implemented yet." if $VERBOSE
#       return []
#     end

#     # isJust		-- :: Maybe a -> Bool
#     def isJust
#       warn "Method 'isJust' is not implemented yet." if $VERBOSE
#       return []
#     end

#     # isNothing		-- :: Maybe a -> Bool
#     def isNothing
#       warn "Method 'isNothing' is not implemented yet." if $VERBOSE
#       return []
#     end

#     # fromJust		-- :: Maybe a -> a
#     def fromJust
#       warn "Method 'fromJust' is not implemented yet." if $VERBOSE
#       return []
#     end

#     # fromMaybe		-- :: a -> Maybe a -> a
#     def fromMaybe
#       warn "Method 'fromMaybe' is not implemented yet." if $VERBOSE
#       return []
#     end

#     # listToMaybe       -- :: [a] -> Maybe a
#     def listToMaybe
#       warn "Method 'listToMaybe' is not implemented yet." if $VERBOSE
#       return []
#     end

#     # maybeToList	-- :: Maybe a -> [a]
#     def maybeToList
#       warn "Method 'maybeToList' is not implemented yet." if $VERBOSE
#       return []
#     end

#     # catMaybes		-- :: [Maybe a] -> [a]
#     def catMaybes
#       warn "Method 'catMaybes' is not implemented yet." if $VERBOSE
#       return []
#     end

#     # mapMaybe		-- :: (a -> Maybe b) -> [a] -> [b]
#     def mapMaybe
#       warn "Method 'mapMaybe' is not implemented yet." if $VERBOSE
#       return []
#     end

#   end # Maybe

end # Prelude